[arm] Use SmallVector for pending constant pool

Since there usually aren't many pending 32-bit constants, we can avoid
the cost of malloc/free for most compiles by using a base::SmallVector
instead of std::vector.

I picked a value of 32 entries as the SmallVector's inline size, based
on compiling three.js and jquery with Sparkplug, and printing the size
of the constant pool when emitted. 93% of emitted constant pools had
<=32 entries.

Bug: v8:11420
Change-Id: I7d62da74c60feae08f8a4b16b1e7f93ea69d2c95
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2840447Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74092}

src/codegen/arm/assembler-arm.cc

@@ -534,7 +534,6 @@ Assembler::Assembler(const AssemblerOptions& options,
     : AssemblerBase(options, std::move(buffer)),
       pending_32_bit_constants_(),
       scratch_register_list_(ip.bit()) {
-  pending_32_bit_constants_.reserve(kMinNumPendingConstants);
   reloc_info_writer.Reposition(buffer_start_ + buffer_->size(), pc_);
   next_buffer_check_ = 0;
   const_pool_blocked_nesting_ = 0;
@@ -5267,7 +5266,7 @@ void Assembler::ConstantPoolAddEntry(int position, RelocInfo::Mode rmode,
     }
   }
 
-  pending_32_bit_constants_.push_back(entry);
+  pending_32_bit_constants_.emplace_back(entry);
 
   // Make sure the constant pool is not emitted in place of the next
   // instruction for which we just recorded relocation info.

src/codegen/arm/assembler-arm.h

@@ -45,6 +45,7 @@
 #include <memory>
 #include <vector>
 
+#include "src/base/small-vector.h"
 #include "src/codegen/arm/constants-arm.h"
 #include "src/codegen/arm/register-arm.h"
 #include "src/codegen/assembler.h"
@@ -1153,8 +1154,8 @@ class V8_EXPORT_PRIVATE Assembler : public AssemblerBase {
   // PC-relative loads, thereby defining a maximum distance between the
   // instruction and the accessed constant.
   static constexpr int kMaxDistToIntPool = 4 * KB;
-  // All relocations could be integer, it therefore acts as the limit.
-  static constexpr int kMinNumPendingConstants = 4;
+  // Experimentally derived as sufficient for ~95% of compiles.
+  static constexpr int kTypicalNumPending32Constants = 32;
   static constexpr int kMaxNumPending32Constants =
       kMaxDistToIntPool / kInstrSize;
 
@@ -1258,7 +1259,8 @@ class V8_EXPORT_PRIVATE Assembler : public AssemblerBase {
   // pending relocation entry per instruction.
 
   // The buffers of pending constant pool entries.
-  std::vector<ConstantPoolEntry> pending_32_bit_constants_;
+  base::SmallVector<ConstantPoolEntry, kTypicalNumPending32Constants>
+      pending_32_bit_constants_;
 
   // Scratch registers available for use by the Assembler.
   RegList scratch_register_list_;